1. Field of Invention
The present invention relates to technology for constituting a resonance type of longitudinal double-mode SAW filter obtained by utilizing surface acoustic waves, wherein when the device is formed on a piezoelectric plate that is used, a reflection coefficient γ of the surface acoustic waves possessed by an electrode conductor can be as large as about 0.1.
2. Description of Related Art
Resonance type longitudinal double-mode SAW filters having two interdigital transducers (hereinafter abbreviated as IDTs)) and three IDTs have been disclosed in JP-A-61-285814 and JP-A-1-231417, respectively.
According to the above in which a piezoelectric plate is fabricated, for example, with ST-cut quartz or STW cut, however, the insertion loss of the longitudinal double-mode SAW filter greatly increases as the operation frequency of the device exceeds 800 MHz. In particular, when a longitudinal double-mode SAW filter is to be realized for VCSO use at 2.5 GHz by utilizing the surface acoustic waves of the STW (surface transversal wave) type with the STW cut, it often happens that the reflection coefficient γ of the device per an electrode finger tends to become as great as 0.06 to 0.16 (see FIG. 2). In this state, the number of IDT pairs, M becomes as small as 60 to 80. In order to realize a filter having an impedance of 50Ω, therefore, the ratio of intersection widths of the electrode fingers of IDTs becomes as great as 130 wavelengths causing an increase in the resistance of the electrodes, exhibiting pass characteristics of a single peak, an insertion loss of as large as 8 to 12 dB to deteriorate performance, and making it difficult to obtain a desired bandwidth.
Specifically, FIG. 9 illustrates an example of transmission characteristics possessed by a conventional longitudinal double-mode SAW filter, wherein the abscissa is an axis of frequency that indicates a change of frequency df/f (ppm) standardized by a center frequency of a filter, and the ordinate that indicates the operation transmission quantity Sb (dB). Reference numeral 902 denotes a problematic pass characteristics of a single peak shape. The 3-dB bandwidth is about 200 ppm, which is not sufficient for the VCSO use. The insertion loss shows a minimum value Sbmin of SB(f) in the transmission characteristics 901. Experiment shows that the insertion loss Sbmin tends to increases as the 3-dB bandwidth decreases.
FIG. 2 is a diagram illustrating the calculated results of when the ratios H/λ of the wavelength λ of surface acoustic waves and the thickness H of the electrode film, when H/λ are 0.03 and 0.05 on the STW substrate on a crystal where azimuth angles of the piezoelectric plate is (φ, θ, ψ)=(0, 130±5, 90) in Euler angles. Abscissa indicates the ratio of line widths η=L/P between the line width of the electrode L and P=S+L that is the sum of the width L of the electrode and a space S. The ordinate indicates the reflection coefficient γ of the surface acoustic waves per an electrode. Here, numerals in parentheses are those of when the device frequency is 2.5 GHz, and are 61.5 nm when H/λ is 0.03 and 102.6 nm (10−9m) when H/λ is 0.05. When the ratio η of line widths that are usually used is 0.5, it will be learned from FIG. 2 that the reflection coefficient γ per an electrode finger becomes as great as 0.06 to 0.16.